I mean the application of your definition of optimate/populare should be non-controversial.
As an example, one might take the man's position on tribunician power to be the defining feature of the optimate/populare categories. Using this definition, one could--without controversy--put Sulla in the optimate category (since he sponsored the laws depriving them of power) and put Pompey and Crassus in the populare category (since they sponsored the laws that returned power to tribunes). An example of controversial placement would be the categorization of people who had no known position on the power of tribunes (e.g., Catulus).
Some definitions, I think it's easy to see, would be inherently controversial to apply. The notion that the 'optimates' were the "good men", for example, couldn't be applied without controversy. After all, was Cicero a good man? Obviously the matter is controversial. Alternatively, the notion that the 'populares' were "for the people" also couldn't be applied without controversy since everyone claimed to be "for the people". (Recall Nasica's contio in which he told the crowd, "Silence, please, Citizens; for I know better than you what is good for you."!)